- Concrete Polishing Equipment
- Get the Look - Polished Concrete Pictures
- How to Polish Concrete: Learn the basics and get a step-by-step overview
- Polished Concrete Preparation: How to clean, repair, and evaluate before polishing
- Wet vs. Dry Concrete Polishing: What's the difference?
- Concrete Polishing vs. Resurfacing
- Reviews of Concrete Polishing Equipment How to clean, repair, and evaluate the condition of concrete floors before polishing
- Tips on Using Polishing and Grinding Equipment
- Tips for Choosing Edge Grinders
- Selecting Dust-Collection Equipment
- Diamond Tooling: Tips for choosing the right type of diamond tooling and equipment
- Concrete Densifiers: An introduction to chemical hardeners and how they work to improve polished concrete
- Buying Tips for Polishing Equipment: Before you invest in a grinder, be sure to listen, learn and ask the right questions
How to Polish ConcreteAn overview of polishing basics and the step-by-step process
As with any other specialized technique, polishing is a multi-step process requiring use of the proper tools and equipment to achieve top-quality results. To help you get started, here's a brief overview of the polishing process and a checklist of basic equipment and supply needs, along with some buying tips. Note that each job will present different conditions and challenges, so be sure to consult with your equipment and material suppliers for recommendations as to the products best suited for your application.
Polishing concrete is very similar to sanding wood. Machines equipped with diamond-segmented abrasives (akin to sandpaper) are used to grind down concrete surfaces to the desired degree of shine and smoothness. As when sanding wood, you gradually progress from coarser-grit to finer-grit abrasives. (In this case, grit is the particle size of the diamond.) The result is a glossy, mirror-like finish.
You can polish concrete using wet or dry methods. Although each has its advantages, dry polishing is the method most commonly used in the industry today because it's faster, more convenient, and environmentally friendly. Wet polishing uses water to cool the diamond abrasives and eliminate grinding dust. Because the water reduces friction and acts as a lubricant, it increases the life of the polishing abrasives. The chief disadvantage of this method is the cleanup. Wet polishing creates a tremendous amount of slurry that crews must collect and dispose of in an environmentally sound manner. With dry polishing, no water is required. Instead, the floor polisher is hooked up to a dust-containment system that vacuums up virtually all of the mess.
SUMMARY OF BASIC POLISHING STEPS
- Remove existing coatings (for thick coatings, use a 16- or 20-grit diamond abrasive or more aggressive tool specifically for coating removal, such as a T-RexTM).
- Seal cracks and joints with an epoxy or other semi-rigid filler.
- Grind with a 30- or 40-grit metal-bonded diamond.
- Grind with an 80-grit metal-bonded diamond.
- Grind with a 150-grit metal-bonded diamond (or finer, if desired).
- Apply a chemical hardener to densify the concrete.
- Polish with a 100- or 200-grit resin-bond diamond, or a combination of the two.
- Polish with a 400-grit resin-bond diamond.
- Polish with an 800-grit resin-bond diamond.
- Finish with a 1500- or 3000-grit resin-bond diamond (depending on the desired sheen level).
- Optional: Apply a stain guard to help protect the polished surface and make it easier to maintain.
POLISHED CONCRETE STANDARDS
There are not published standards for polished concrete, but it is generally agreed that the concrete must be polished through the sequence of disks ending with 1800-3500 grit diamonds to be considered polished concrete. At this level the concrete will exhibit a glossy sheen and high reflectivity without the use of a topical coating.
Polished concrete in not simply exposing the rock in the concrete mix then applying a sealer.
During the polishing process an internal impregnating sealer is applied. The sealer sinks into the concrete and is invisible to the naked eye. It not only protects the concrete from the inside out, it also hardens and densifies the concrete. This eliminates the need for a topical coating, which reduces maintenance significantly (versus if you had a coating on it).
When Demmert and Associates brings the concrete to a full polish, they never apply a topical coating or wax. According to Greg Demmert, "Waxing the surface would be defeating the purpose of a fully polished floor because the concrete floor itself is already shiny, so there is no need to put something on the floor that would then need to be maintained."
A TEAMWORK APPROACH TO POLISHED CONCRETE
Recently I was asked which changes or advancements have had the biggest impact on polished concrete floors, primarily in the industrial and commercial market sectors. Although we could discuss equipment improvements or how diamond tooling technology has significantly advanced through the years, the biggest impact by far is how concrete slabs are poured and treated from the ground up.
This first became apparent seven years ago, when a high-end client engaged our services to evaluate their polished concrete program. While touring four of their stores that had been in service for nearly a year, it was immediately obvious to us that the floors were not being poured flat due to the random pockets of coarse aggregate exposure. To improve the overall quality of the polishing, we needed to start by improving the quality of the concrete placement, especially how the slabs were being finished, cured and protected during the construction phase.
Today, there is now a heightened awareness of all of the team players involved in producing these floors, and it has dramatically helped to improve the overall quality of polished concrete. That said, it can still be very challenging to work with the various trades and recommend that they perform their work in a way that deviates from the norm and their standard routine.
A new game plan
Concrete finishers can be notorious for being set in their ways, and often they don’t like outside sources asking them to finish a slab a certain way. Being a third-generation finisher myself, I appreciate this mentality and I love how most finishers consider themselves artists and take an attitude of ownership of the slab they are finishing. Finishing crews are accustomed to burnishing the slab to the point of darkening the concrete and obtaining a high shine, and they will often wet cure the slab with blankets for three to four days to increase the overall strength of the floor. This combination of wet curing and a burnished slab can wreak havoc with polishing contractors, who will typically need to use their most aggressive diamond tooling combined with wet cutting to expose the appropriate amount of aggregate. We have found that producing a “haze” or “smoke” finish, with little if any sheen or burnishing, is more conducive for initial grinding. But this must be done without compromising floor flatness and levelness numbers, as slab placement and finishing directly impacts the quality of the polishing.
Winning strategies for polished floors
While working on a project with Denny Bartz of Structural Services Incorporated, I asked about the dynamics of industrial slab placements specifically for the intent of polishing. Here are some strategies that he recommends for producing high-quality polished floors:
Go back to the basics
Harris: With recent improvements on finishing slabs for polished concrete, have you seen the mix design change?
Bartz: We have reverted back to simplified mix designs, where in the past specifications called for ultra-high-performance mixes consisting of many different admixtures. By getting back to the basics and specifying well-graded mixes, we are able to produce a uniform slump which minimizes differential setting times of the concrete which helps in producing a much more consistent finish without surface mottling.
Control the playing field
Harris: I know you are overseeing slab placements in many different environments, including harsh temperatures. What are a few considerations?
Bartz: Controlling the placing environment can be one of the most challenging considerations when you take into account subzero temperatures on some slab placements. Obviously this means controlling the concrete temperatures from the batch plant to the jobsite. Also, it is mandatory to maintain a consistent minimum ambient and subbase temperature of 55° F while the concrete needs to be a minimum of 60° F at the point of placement. Since temporary heating is being used, proper ventilation is a must and air quality is continually being monitored.
Harris: Have you found it to be challenging to recommend to the concrete placement crew to finish the slab a certain way?
Bartz: Sometimes yes and sometimes no. I have found that most finishing crews are receptive to recommendations at the preslab placement meetings. Occasionally, I will come across a crew that has been finishing for years and does not embrace change with open arms. Typically, finishers are told to burn a floor in as tight as possible without scratching the surface, which is not the most desirable finish if the intent is to polish it. To eliminate mottling, smears and other surface imperfections, a less aggressive, lighter finish using minimal hand tooling appears to produce a better canvas for a homogeneous polished surface.
Pay attention to the details
Harris: I have seen the methodology on how theses slabs are being cured change over the years. What are your thoughts on the curing of concrete slabs specifically for polished concrete?
Bartz: Choosing a curing product and method that minimizes or eliminates surface discoloration has been a huge challenge. Originally we thought liquid-applied curing membranes were the most effective. However, on several projects, the applicators were not using the proper tip, which created drips and speckles over the entire slab. Considering the liquid was not atomized at the time of application, the heavier areas of the curing compound resulted in differential cure. Even after grinding, we still ended up with darker speckles across the slab. The end result was the polished concrete contractor needed to grind deeper to remove the blemishes even though the project was not originally bid to grind that deep. For the last year or so, we have been using the wet cure method with curing blankets left on the slab for three days on the polished concrete areas. This system has seemed to improve the other issues. However, if it is not installed properly (stretched tight and wrinkle free), you can be left with blanket lines as a result of the blanket not being uniformly in direct contact with the concrete.
The importance of a preslab construction meeting
As you can see, it takes a lot more than just the polishing contractor to produce a stunning polished concrete floor. Many clients are now implementing preslab construction meetings and holding the entire construction team accountable for everything from slab placement to the time the store opens. This has dramatically improved the final appearance of the polished concrete.
If you have never attended a preslab construction meeting specific to the polishing of concrete floors, you’d be surprised at the scope of the participants who attend. In addition to the key players, including the owner’s representatives, general contractor, concrete subcontractor, and polishing contractor, other members of the team that can have an impact are present, from the ready-mix supplier and concrete testing agency to plumbers and electricians.
At these meetings, which typically last several hours, many topics are covered in great detail, all the way down to the specifics of how the slab will be protected both before and after polishing:
- Pour schedule
- Test panel location and requirements
- Placement environment
- Soil support system
- Concrete materials and specific mix design
- Concrete testing and observation
- Floor flatness/Floor levelness requirements
- Slab reinforcement
- Joint detail
- Placing and finishing procedures
- Moisture testing
- Joint filling and materials
- Polished concrete surface
- Slab protection, pre and post polish
It is now mandatory for GCs to submit a pre- and post-slab protection plan and have all of the subs sign off on the sheets during their weekly meetings.
DEFINING RESPONSIBILITIES ON A COMMERCIAL POLISHING PROJECT
As a polished concrete consultant, I have been involved recently in several preconstruction meetings for a large retail store chain.
The clients chose polished concrete for a good portion of their interior floors for a variety of reasons, including aesthetics, ease of maintenance, and light reflectivity. Upon visiting some of their earlier polished floors, we noticed a few problems that ultimately had nothing to do with the quality of the work done by the polishing subcontractor, but instead resulted from the work of the placing and finishing contractor. When polished concrete started becoming popular, specifications were being written inappropriately, setting up the polishing contractor for failure. One specification we came across in particular called for a light salt-and-pepper finish (exposing the sands with no coarse-aggregate exposure), but made no reference to floor flatness(FF) or floor levelness (FL) tolerances. As a result, some of these floors exhibited unlevel, wavy surfaces with raised sections that made it nearly impossible to expose only the sands. These areas showed patches of coarse-aggregate exposure, which the owners did not want. The remedy for this problem was to install the concrete within specified tolerances. In addition, specifying lower-shrinkage concrete materials, combined with tighter joint spacing, would significantly reduce the amount curling the concrete would exhibit.
Witnessing these recurring problems, it became apparent that we needed to revisit the entire concrete placement process, starting with how the subgrade was prepared all the way through to the curing of the slab and, ultimately, the polishing. After attending several of these preconstruction brainstorming sessions, it really got me thinking about the crucial roles each of the different trades play during the concrete construction process. Everyone involved on a polishing project should be sharing the same common goal, which is to create a durable, architecturally pleasing floor. What's more, these floors have to be reproducible on a consistent basis from one region to the next. Of course, this is a daunting task considering some of the jobsite variables involved.
As you can see, there's a lot more to a successful polishing project than simply passing a grinding machine back and forth across the concrete. Each trade needs to contribute their specific knowledge and skills and be held accountable for their scope of work.
CHECKLIST OF RESPONSIBILITIES ON A COMMERCIAL POLISHING PROJECT
Here is an overview of some of the key issues each contractor should be held accountable for on a polishing project. The purpose of this list is not to hang anyone out to dry if something goes wrong, but rather to let all parties know what is expected of them and that they need to strictly adhere to the project specifications to avoid potential problems.
Placing and finishing contractor
- Determining how the concrete will be delivered. Should it be tailgated (poured directly out of the truck) or pumped?
- Protecting the fresh concrete under extreme conditions. If the concrete can potentially freeze, measures should be taken to keep the air warm inside the building when the trucks enter and exit, such as by setting up temporary tented rooms with large flaps that open and close.
- Checking that jobsite water (water of convenience) is only added one time to the concrete by a preapproved person, per the specification requirement.
- Verifying that the inside ambient temperature of the building is a minimum of 55° F.
- Installing the concrete at the specified thickness and then laser screeding and finishing the surface to ensure that the floor meets the specified flatness and levelness requirements.
- Cutting contraction joints in the slab at the specified depth and spacing, using an early-entry saw that won't mar the surface and then removing any residual dust.
- Making provisions to properly cure the slab. Should the slab be wet cured or should a sacrificial liquid membrane be applied? Wet curing is not always practical because it can take much longer for the slab to reach acceptable moisture-vapor readings.
Independent testing firm
- Monitoring the subgrade to make sure the loaded concrete trucks are not creating ruts deeper than ½ inch.
- Testing the concrete in the first truck for slump, temperature, air content, and unit weight and then checking roughly every fifth to tenth truck thereafter. Test cylinders should also be taken.
- Verifying that the concrete was placed and finished to the specified floor flatness (FF) and levelness (FL) tolerances.
- Checking the vapor barrier and reinforcing steel before concrete delivery. Are all the seams in the vapor barrier properly sealed? Is the rebar consistently tied on 18-inch centers and uniformly lifted and chaired in position in the middle of the slab?
- Making sure that when the concrete truck backs over the steel, it is not bent and the vapor barrier is not punctured.
- Checking isolation joints to ensure that all steel columns are wrapped with closed-cell foam and there is no concrete-to-steel contact.
- Checking construction joints to confirm that ¼-inch plate dowels are located exactly middepth of the slab on 24-inch centers.
- Performing a mockup once the concrete slab has cured sufficiently so the owner or owner's agent can sign off on the dye/stain color, the quality of the finish, the color of the joint filler, the topical protective treatment, and the degree of shine. (This should be done roughly 30 days after concrete placement and after the HVAC system has been operating for a minimum of 14 days.)
- Filling the joints with the approved filler material.
- If areas need repair from poor finishing or damage during construction, using the appropriate repair material that matches the color of dye or stain to be used.
- Taking gloss meter readings to make sure the concrete meets the specified minimum gloss requirements after polishing.
Other building trades
- During the remaining construction, making sure all equipment on the floor is equipped with nonmarking tires and is diapered to prevent oil and hydraulic stains.
ADDING DESIGNS AND GRAPHICS TO POLISHED CONCRETE
Before you consider taking your polished concrete floors to the next level including designs and graphics, refine and perfect the basics discussed in this article. Avoid cutting corners or skipping steps. After all, there is no substitute for doing a job the right way.